1. Field of the Invention:
This invention relates to transformers for use in static inverters. Static inverters are devices in which electrical energy is converted to electrical energy in another form through static or non-moving parts. In static inverters to which the present transformer has application, a dc source produces a current through one, two or four semiconductor devices, each connected in series with a transformer winding, the arrangement producing a transformed output as the semiconductor devices are switched on and off. The transformers typically include control windings for efficient and stress-free switching of the associated semiconductor devices.
2. Description of the Prior Art:
Transformers for the static inverter application have been described in the U.S. Pat. Nos. 3,914,680; 4,002,999 of Hesler et al, and in the more recently filed application Ser. No. 875,337 of Peil et al, now abandoned and refiled as Ser. No. 28,405, now U.S. Pat. No. 4,259,716, all of which are assigned to the Assignee of the present application. In these citations, a transformer is described with properties which have been tailored to operating a transistor at a switching rate above audible frequencies (&gt;15 KHz). A property of each of these transformers is to maximize the switching efficiency of the transistor and to avoid unduly stressing it. Stressing typically occurs from a decrease in impedance as seen by the transistor which results when the core becomes fully saturated. In each of the cited cases, a means is provided for determining when a specified level of flux density has occurred and then turning off the transistor before full core saturation and the detrimental stresses can occur. In the devices disclosed, positive, conduction inducing feedback is provided before attaining a specified level of flux density, and then negative, conduction inhibiting feedback is provided after this level of flux density is attained. In all of the arrangements, an aperture is provided through which control windings are threaded and these are typically connected between base and emitter electrodes of the associated transistors. The position of the aperture in the core defines in part the specified level of flux density at which the base drive will reverse.
While the foregoing arrangements share the preceding properties, there are significant differences. The devices described in U.S. Pat. No. 4,002,999 contain a single loop core (e.g. that made from two "C" cores) in which a single aperture is provided through which a figure "8" secondary control winding is threaded. A control winding serially connected with the main winding is also provided for predisposing the core on one side of the aperture to saturate first. When the pre-disposed side of the core saturates, the feedback induced in the figure "8" winding changes from conduction inducing to conduction inhibiting. The device acts as a current transformer prior to partial saturation and as a voltage transformer subsequent to partial saturation. The two modes are mutually exclusive and the transition is immediate.
Another transformer is described in U.S. application Ser. No. 28,405, filed Apr. 9, 1979, now U.S. Pat. No. 4,259,716, a continuation in part of application Ser. No. 875,337 filed Feb. 6, 1979, now abandoned. In that application, a single path core, typically made from the "C" cores (but also using shunted cores) is employed. Instead of a single aperture, two apertures are provided aligned along the direction of the main flux. A secondary control winding wound through the apertures is uncoupled to the main flux and is in a "neutral" region. A control winding energized simultaneously with the main winding is wound through the two apertures so as to provide a flux circulating about both apertures in respectively clockwise and counterclockwise modes. The circulating flux adds and subtracts from the main flux in five regions defined around the apertures. In the absence of saturation, the main flux is not coupled to the secondary control winding and the secondary control winding responds only to current passing through the primary control winding. However, as one or more regions saturate, the main flux is eventually steered through the neutral fifth region where it couples to the secondary control winding and reverses the sense of the feedback. The two aperture structures permits one to program the turn-off sequence by introducing intermediate stages in which the conduction inducing feedback is reduced and/or terminated prior to the application of conduction inhibiting feedback. The two-aperture configuration is a current transformer prior to partial saturation and a current transformer of reverse sense subsequent to saturation. With a transistor load, an alternative mechanism of drive reversal may occur, in which current transformer action continues before and after partial saturation.
A disadvantage of the two aperture configuration is the cost of making a second aperture, and the cost dictated requirement that the two apertures lie in a segment of the core structure in which all of the flux flows. With single loop cores or with a shunted core, the main windings and control windings can be placed on separate legs without mutual interference. With figure "8" cores, however, the main flux pursues two loops which consolidate in the central leg. The two aperture arrangement needs to be located on the central leg where all available space is most efficiently allocated to the main windings. Since figure "8" cores have a more efficient core utilization factor, it has been the object of the present invention to devise a transformer having the basic properties of the two aperture device, but one which would provide a minimum of interference between the control and the main power windings.